1, 2. Idea,thought,conception,notion refer to a product of mental activity. Idea, although it may refer to thoughts of any degree of seriousness or triviality, is commonly used for mental concepts considered more important or elaborate: We pondered the idea of the fourth dimension. The idea of his arrival frightened me.Thought, which reflects its primary emphasis on the mental process, may denote any concept except the more weighty and elaborate ones: I welcomed his thoughts on the subject. A thought came to him.Conception suggests a thought that seems complete, individual, recent, or somewhat intricate: The architect's conception delighted them.Notion suggests a fleeting, vague, or imperfect thought: a bare notion of how to proceed.4. sentiment, judgment.

idea

n.

late 14c., "archetype of a thing in the mind of God; Platonic `idea,'" from Latin idea "idea," and in Platonic philosophy "archetype," from Greek idea "ideal prototype," literally "the look of a thing (as opposed to the reality); form; kind, sort, nature," from idein "to see," from PIE *wid-es-ya-, suffixed form of root *weid- "to see" (see vision). Sense of "result of thinking" first recorded 1640s.

Men of one idea, like a hen with one chicken, and that a duckling. [Thoreau, "Walden"]